Tunable Hidden Altermagnetic Spin Splitting in Layered Ruddlesden–Popper Oxides

Altermagnets (AMs) are unconventional collinear antiferromagnetic materials that have recently been discovered to exhibit nonrelativistic spin splittings despite their fully compensated magnetization. Leveraging the advantages of ferromagnets and conventional antiferromagnets, AMs offer great potential for high-density, high-frequency spintronic devices. Combining symmetry analysis and first-principles calculations, we show that such altermagnetic spin splittings exist locally in layered Ruddlesden-Popper oxides (e.g., Ca₂MnO₄ and La₂NiO₄) but are ultimately hidden when there are an even number of perovskite layers. We demonstrate that the local spin splitting can be made globally apparent via an electric field effect, which breaks inversion symmetry. Furthermore, we demonstrate the tunability of altermagnetic properties by oxygen stoichiometry engineering with equatorial oxygen vacancies enhancing the spin splitting. A large concentration of apical oxygen vacancies further drives an insulator-to-metal transition. Our work not only broadens the AM materials platforms but also provides strategies for tuning the electronic structure for antiferromagnetic spintronic applications.